Perpendicular recording has been developed in part to achieve higher recording density than is realized with longitudinal recording devices. A PMR write head typically has a main pole layer with a small surface area at an ABS, and coils that conduct a current and generate a magnetic flux in the main pole that exits through a write pole tip and enters a magnetic media (disk) adjacent to the ABS. The flux is used to write a selected number of bits in the magnetic media and typically returns through a shield structure to a back gap region of the write head which connects the main pole with the return shield. The return shield may also serve as the top shield in a read head formed below the write head in a combined read-write structure.
Perpendicular magnetic recording has become the mainstream technology for disk drive applications beyond 150 Gbit/in2. The demand for improved performance drives the need for a higher areal density which in turn calls for a continuous reduction in transducer size. A PMR head which combines the features of a single pole writer and a double layered media (magnetic disk) has a great advantage over LMR in providing higher write field, better read back signal, and potentially much higher areal density. Current magnetic heads generally consist of a writer and a reader that are formed adjacent to one another along an ABS. The read head may be based on a TMR element in which a tunnel barrier layer separates two ferromagnetic (FM) layers where a first FM layer has a fixed magnetization direction and the second FM layer has a magnetic moment that is free to rotate about a direction orthogonal to the direction of the magnetic moment in the reference “fixed” layer. The resistance across the barrier changes as the free layer moment is rotated. This signal is used to detect the small magnetic field from the recorded magnetization pattern on the media.
Reducing the magnetic spacing from read/write heads to the magnetic media during both writing and reading is the most important factor in achieving better performance in high density recording. The writer and reader are separated by several microns in a typical recording head and are made of several different materials each having a unique coefficient of thermal expansion (CTE). Therefore, the protrusion of the reader and writer are usually quite different due to the effect of varying operating temperatures, applying dynamic flying height (DFH) power to actuate the reader or writer, or from write current excitation. In addition, the point with minimum spacing to disk could be located away from either the reader or the writer, imposing further restrictions to achievable magnetic spacing during reading and writing. Improvements in PMR head design are needed to control the protrusion differences at the writer, the reader and the minimum point, and its variation. In particular, for the touchdown and then back off mode of operation using DFH, if the writer protrusion is much more than the reader protrusion, then the minimum reader spacing is determined by the excess protrusion plus any initial protrusion. The ratio of reader protrusion rate/writer protrusion rate is called the gamma ratio. A lower gamma ratio means the writer protrusion rate is much higher than the reader protrusion rate, and could potentially put a greater limit to achievable reader spacing.
While the ABS surface of the magnetic recording head protrudes at reader and writer positions during operation, the reader is being heated by the DFH. This is needed as thermal energy is the driving force to induce elastic deformation for enabling the reader to be at close proximity to the media for enhanced read back signal amplitude and signal-to-noise ratio (SNR). However, the reader is preferably kept at a cooler temperature due to a concern for high temperature noise (HTN) and instability issues. With TAMR recording as the post-PMR technology to push areal density even higher, the reader temperature rise budget becomes even tighter when including the heating associated with laser diode and waveguide operation. A novel recording head structure is desirable to reduce the reader temperature rise during operation while still enabling the reader to be near the touchdown location to minimize reader to media spacing. As a measure of Figure of Merit (FOM); the DFH induced reader heating can be quantified by the temperature rise in the reader per unit of actuation (nm) delivered by the DFH.
As the areal density requirement for magnetic recording becomes more stringent and dictates the performance of hard disk drives (HDD), magnetic spacing for both reader and writer is pushed close to the limit. The practice of energizing DFH to induce elastic protrusion of the ABS for touching down on the media has been improved significantly with the aid of fully integrated Head Disk Interference (HDI) and acoustic emission (AE) sensors. After determining the touch down of the head onto a disk, the DFH power is then reduced to enable the back off of the reader spacing at operation following the Wallace Spacing Equation. The back off amount has gradually diminished from generation to generation of devices and has now reached a sub-nanometer distance. Dynamic control of reader spacing involves a thin layer of heater film that is embedded inside the magnetic recording head, and usually within one or more insulation layers. The joule heating from the electrical current into the heater film is conducted away from the source to the entire slider body.
The Wallace Spacing Equation published by R. L. Wallace is in “The reproduction of magnetically recorded signals”, Bell Syst. Techn. J., 30, 1145-73 (1951). A key point is that the read back amplitude from a single frequency pattern on a disk decays exponentially when the sensor to disk spacing is increased. The Wallace Spacing Equation is represented by A=A0*exp(−d*2π/l) where “l” is the period (in a unit length such as nm), A is the read back amplitude, and “d” is the spacing between the read head sensor and the disk when the sensor is designed such that the intrinsic spacing resolution is better than the recording pattern period.
A common way to increase the read gap (RG) actuation during DFH operation is to increase the lower read shield (S1) thickness. The increased volume of the S1 shield enables the RG to protrude more at the same power, thus improving the RG gamma parameter and dynamic performance (DP). The thickness of the upper read shield (S2A) is part of the contribution to reader-writer separation which is desired to be as small as possible in order to have high format efficiency in the drive. With the increased imbalance of the S1 and S2A shield thicknesses resulting from a thicker S1 shield, certain drawbacks in magnetic characteristics associated with the read shield thickness ratio create undesirable transfer curves for the reader. One drawback is an increased hysteresis reject rate during quasi-static (QST) testing. In addition, a thicker S1 shield reduces the QST amplitude for a fixed field span testing. This indirectly impacts QST based noise testing such as PAT (proportional amplitude testing). Although this issue can be addressed in principle by new testing conditions, significant investment would be required for appropriate tester upgrades. An alternative to a thicker S1 layer is desirable in order to improve RG actuation without adversely compromising other read head characteristics. Thus, an improved read head design is needed to enable further advances in magnetic recording technology. The read head structure is further required to be robust against wear from mechanical stress induced at touchdown calibration and potential HDI events during the HDD operation throughout its lifetime.